In order to increase the use efficiency of light spectra, multilevel modulation such as QAM (quadrature amplitude modulation) and OFDM (orthogonal frequency division multiplexing) is being variously examined.
One of the methods for obtaining a multilevel optical signal is to drive a push-pull drive Mach-Zehnder modulator (MZM) using a multilevel electric signal.
FIG. 1 illustrates a conventional push-pull drive MZM 100. The example illustrated herein is a single-ended electrode type MZM using an X-cut lithium niobate (LiNbO3) substrate. In FIG. 1, the MZM 100 includes: a Mach-Zehnder interferometer-type optical circuit including a light splitting portion 101 and a light combining portion 102; a traveling wave-type modulating electrode 103; and a DC bias electrode 104 of lumped parameter-type. For simplifying the drawing, each electrode is illustrated with only a signal line, and the ground electrode is not illustrated. An optical signal propagating each optical waveguide is given phase shifts of +φ and −φ by a driving electric signal inputted to the modulating electrode 103. Herein, φ=(π/2Vπ)·V where V is a voltage level of the driving electric signal and Vπ is a voltage to change the relative optical phase between the arms by π. The optical signal propagating each optical waveguide is further given a phase difference of π by bias voltage applied by the DC bias electrode 104. Herein, the MZM light electric-field response is represented by sin φ.
FIG. 2 illustrates a response curve of the electric field of the output optical signal to the driving voltage in the conventional MZM. As shown in FIG. 2, in the conventional MZM, the response curve with respect to the driving voltage is non-linear. Accordingly, when the MZM is driven by a multilevel electric signal, the output optical signal is shifted from an ideal and equal interval output optical signal obtained when the response curve is linear.
On the other hand, if the amplitude of the driving voltage is reduced from 2Vπ in order to reduce the signal distortion, large optical loss is generated as illustrated in FIG. 3 (see NPL 1).